It is heresy even to pose the question to the chancellor's supporters before Sunday's general election and, if she does win a third term, the answer will remain evasive. But some Germans are asking anyway: who can succeed Angela Merkel?

"Angela the Great", as one magazine dubbed her, looks certain to be re-elected, the main doubt being whether her centre-right coalition gets a new lease of life or she has to govern with the centre-left. Both scenarios are fine with the public, pollsters say, as long as "Angie" stays in charge.

But two credible media sources recently reported that she will serve only two or three years of any new four-year term. The reports, denied by Merkel, will make it hard for her conservative Christian Democratic Union (CDU) party to keep a lid on succession talk for long.

Three potential heirs are widely touted - Labour Minister Ursula von der Leyen, Lower Saxony's former premier David McAllister and Defence Minister Thomas de Maiziere - though all deny any such ambition. None has Merkel's command of the party, or her ability to glide over daily politics and win elections.

Smiling benignly from giant posters or offering reassurance in bland campaign speeches, the popular 59-year-old chancellor leaves it to underlings to debate policy details with struggling Social Democrat (SPD) challenger Peer Steinbrueck.

A majority of Germans say they feel comfortable under her steady, homely leadership, and many are in no hurry to know who comes after her. Party leaders avoid the issue publicly.

"Either Merkel wins the election and there won't be a discussion for a few years, or she loses and the cards will have to be reshuffled completely," said a member of the CDU's top governing board, speaking on condition of anonymity.

Since nobody in the party believes she can lose on Sunday, they are confident the debate "won't get going until a year or two after the election", said another senior party member.

But academics who study the CDU, and some party insiders not in the grip of Merkel mania, believe it would be short-sighted of the party not to start grooming an heir immediately.

Her aides accept succession speculation will be a growing "annoyance factor" in a third term and risks making her a lame duck well before the 2017 election. It must be quashed.

"Of course there is a debate about the succession," said one member of parliament, critical of a cult atmosphere where Merkel has become "our only asset".

"What is the CDU without Angela Merkel? That is the pressing question. When we answer that, a candidate may arise who stands for these values," said the lawmaker, who asked not to be named.

"I really think they need to resolve this sooner than 2017," said politics professor Wichard Woyke, who believes the leadership frenzy will distract the CDU from the business of governing.

"There is no discussion of content. This will be a problem for the CDU in the post-Merkel era," he said.

There are echoes of the CDU under Konrad Adenauer in the early 1960s and Helmut Kohl in the 1990s - two towering figures whose reluctance to make way for an heir caused turmoil in the party and, in Kohl's case, contributed to defeat by the SPD.

Kohl was Merkel's mentor, which did not stop her putting the knife in when, as deputy CDU leader in 1999, she criticised the former chancellor over a party financing scandal.

That act of ruthlessness by Kohl's innocent-looking "maedchen" (girl) put her on a path to power that Wichard said was "strewn with political corpses".

The careers of CDU "princelings" came to premature ends.

Potential rivals were sidelined to jobs in Europe, left politics or were sunk by scandals of vanity or venality. Two ministers fell over plagiarism in academia and Christian Wulff quit as president for taking cheap loans and free hospitality.

This has whittled the succession shortlist down to three Merkel loyalists.

Going by the rule of thumb that the louder politicians deny interest, the keener they are, von der Leyen is the top contender. Dubbed "Mother of the Nation" by one newspaper in reference to her seven children, she is fond of repeating the line that each generation has its leader and "my generation's chancellor is called Angela Merkel".

Younger than Merkel at 54 and in many ways her opposite - a ready smile, blue blood, a western CDU pedigree, cosmopolitan upbringing and vast family complete with dogs and ponies - she ranks close behind veteran Finance Minister Wolfgang Schaeuble in terms of popularity. But she can irritate party colleagues.

"The rule is usually that you either have the party behind you or the public," said the CDU board member. "In von der Leyen's case, it's the public."

Her ministerial experience and progressive profile on issues like gender quotas and same-sex couples, on which Merkel sits on the fence, make von der Leyen a compelling candidate. She may switch after the election to a role with more political heft such as parliamentary floor leader, or even foreign minister.

"The CDU would have to be blind not to pick von der Leyen after Merkel," said social historian Hans-Ulrich Wehler.

David McAllister was wildly popular with public and party until he narrowly lost Lower Saxony state to the SPD this year.

"'Mac' would have been the best successor, which is why the CDU was so shocked by his defeat," said a senior CDU colleague.

The half-Scot, who cried in defeat, is embarking on an EU career but could return as a contender in 2017.

They don't come more loyal than Thomas de Maiziere, Merkel's most trusted lieutenant until a bungled defence contract tarnished his reputation for competence. Aged 59, he lacks charisma: one party colleague described him as "a quintessential bureaucrat".

Two other possibilities must first prove they can win votes: Volker Bouffier, 61, faces a tough election in Hesse on Sunday, while Julia Kloeckner, 41, must win Rhineland-Pfalz in 2016.

Merkel gives no clues except to say "a successor has always been found". The eventual choice may be as improbable as she was - a Protestant woman from former East Germany leading a predominantly Catholic patriarchy from the West.

Her biographer says she may yet try to match Kohl's record 16 years in office by running again in 2017.

"You know I don't decide on things until they crop up," has been Merkel's enigmatic response to that idea.

The message to "wannabes" waiting in line is clear: don't hold your breath.

A Complex Poll

How German elections work:

Two ballots
Germany has a mixed-member proportional voting system under which voters cast two ballots: one directly for a candidate in his or her constituency and the second for a party. This second vote determines the distribution of seats in parliament. Chancellor Angela Merkel's name, for instance, does not appear on the national ballot but only in her constituency in the Stralsund/Ruegen district.

Ballot splitting
German voters sometimes split their ballots to give a preferred coalition extra support. They typically give their first vote to a direct candidate from one of the two main parties - the Christian Democrats (CDU) and sister party the Christian Social Union (CSU) or the Social Democrats (SPD) - and the second vote to the corresponding smaller partner, the Free Democrats (FDP) or the Greens. The FDP and Greens have little interest in the first ballot because Germany's 299 constituencies are won on a first-past-the-post basis, which almost always favours the larger parties. FDP and Greens supporters give the first vote to the CDU/CSU or SPD, while CDU/CSU and SPD backers give their second vote to the FDP or Greens.

Overhang seats
If the CDU/CSU or SPD wins more direct seats in a state than they would get based on their share of second votes, the parliament creates extra "overhang" seats. There are 299 seats in parliament for winners of the direct seats and another 299 seats based on parties' relative strength via the second ballot. The number of overhang seats rose in recent elections because the two main parties, CDU/CSU and the SPD, won all but a handful of the 299 direct seats but had been receiving smaller shares of the second ballots. In 2009, the CDU/CSU won 24 extra overhang seats. A new law compensates smaller parties for overhang seats, thus making it less interesting for the big parties to share support with smaller partners.